Gauge Theory

The standard model of particle physics is based on relativistic quantum gauge field theory. These were first constructed in 1954 by Chen Ning Yang and Robert Mills (so aka Yang-Mills theories), but did not really take-off until their use within Quantum Chromodynamics (QCD) in 1973 by David Gross, Frank Wilczek & David Politzer. The "three forces" can be described as a combination of three Unitary Gauge Groups usually denoted as SU(3)xSU(2)xU(1). SU(3) describes strong interactions with (32-1) = 8 associated massless gauge fields (Gluons) U(1) alone describes electromagnetic interactions with its single massless gauge boson (photon) SU(2) alone does NOT describe weak interactions instead, the mixing of SU(2)xU(1) is necessary to describe electroweak interactions the massless gauge bosons of weak interactions acquire mass by their interaction with a scalar field (the Higgs Field) resulting in a single massless gauge boson (photon as above) 3 (non-zero mass) intermediate vector bosons (W+, W- & Z0) Hence the current model of the mediators of electroweak interactions

Grand Unified Theories (GUTS)

Theories that attempt to unify the electromagnetic weak & strong interactions of the standard model for particle physics in terms of a single, unified interaction. (Note gravitational interactions are excluded). The first theories of this type were proposed in 1974 by Sheldon Glashow & Howard Georgi. The best estimates to-date show that the strengths of all three of these interactions are predicted to be similar (hence the interactions can/might be unified) at energies ~2x1016GeV (Langacker & Polonsky, 1993).

Higgs Field

The GUTs include a set of fields known as Higgs Fields (after Peter Higgs) which enable spontaneous symmetry breaking to occur. There is a Higgs Field for each of the 24 fundamental particles. Each of the fundamental particles is thought of as a bundle of energy of the field. The Higgs Fields are symmetric about their zero points, but the potential has a positive value at this point. Instead the potential is zero at some non-zero value of the fields. The common analogy of the interaction of two Higgs Fields is a "Mexican Hat". See Also: Higgs Fields & Particle Nice Analogy of Higgs Fields & Particles from David Miller (UCL) SciAm

Minimum Supersymetric Extension to the Standard Model (of Particle Physics)

incomplete

Quantum Chromodynamics (QCD)

A theory that offered an explanation of strong interactions between quarks in protons & neutrons in terms of the exchange of gluons QCD introduced the new property of "color" ("red", "green" or "blue") to each quark. Developed in the early 1970s by David Gross, Frank Wilczek & David Politzer.

Quantum Electrodynamics (QED)

A theory combining quantum mechanics and special relativity whereby electromagnetic interactions involved the exchange of virtual photons. Developed in the 1930s and 1940s by Richard Feynman, Shin'ichiro Tomonaga & Julian Schwinger.

Spontaneous Symmetry Breaking

At energies greater than the potential at the zero point of the combined Higgs Fields, there is symmetry. At energies below the potential at the zero point, a particular combination of the fields is randomly chosen. This is random choice is the "spontaneous" breaking of symmetry in GUTs. The apparently distinct properties that we observe for different types of particles today represent the different ways that particles can interact with the Higgs Fields. For instance it is the interaction with the Higgs Fields that determines a particle's mass. Thus after spontaneous symmetry breaking, when a random combination of the fields has been selected, the mass of a particle that interacts with Higgs Field "X" will be different to the masses of particles that interact with Higgs Field "Y", Higgs Field "Z" etc. At high eneries there is no longer any distinction between the various particles which are the interaction mediators. All these mediators behave in the same way, and the strengths of all the "forces" is equal.

Standard Model of Particle Physics

The "standard" model for the fundamental particles and their interaction (but excluding gravtity). The fundamental particles consists of leptons and quarks (hence offering an explanation of mesons & baryons) and the weak, electromagnetic and strong interactions mediated by the gauge bosons.

Jargon from group theory whereby the S stands for "Special condition" U stands for Unitary 2 indicates the order of the group is 2 a mixing of SU(2)xU(1) is necessary to describe the unification of electromagnetic & weak interactions (see Gauge Theory)

SU(3)

Jargon from group theory whereby the S stands for "Special condition" U stands for Unitary 3 indicates the order of the group is 3 SU(3) describes strong interactions (see Gauge Theory)

U(1)

Jargon from group theory whereby the U stands for Unitary 1 indicates the order of the group is 1 (it is superfluous, but retained for generality/consistency). U(1) alone describes electromagnetic interactions. However U(1) & SU(2)interactions offer a description of electroweak interactions. (see Gauge Theory)